CMOS image sensors are devices that convert optical images into electrical signals. A CMOS image sensor may convert signal electrons generated in response to incident light into a voltage signal. Signal processing may be performed on the voltage signal, thereby reproducing image information. CMOS image sensors may include a plurality of MOS transistors, which may be manufactured using a CMOS formation process. The number of MOS transistors generally corresponds to the number of pixels. The MOS transistors may adopt a switching mode to sequentially detect pixel outputs.
Some CMOS image sensors have a relatively simple drive mode in comparison with a charge coupled device (CCD). CMOS image sensors may be implemented in a variety of scanning modes. A CMOS image sensor may be integrated in a single chip together with a signal processing circuit and may be substantially miniaturized. A CMOS image sensor may be manufactured by a compatible CMOS technique with manufacturing costs and power consumption minimized.
If the pitch of a pixel is smaller than 2.25 μm, a 2S-4T (two shared-four transistors) structure may be applied. The 2S-4T structure is developed in order to reduce the pitch of a pixel and enlarge the aperture ratio of a photo diode. In the 2S-4T structure, two pixels, each having four transistors, may be combined in a unit cell and three transistors may be commonly used by two photo diodes.
Example FIG. 1 is a plan view showing a layout of an image sensor with a 2S-4T structure in accordance with the related art. Example FIG. 2 is a plan view exemplarily illustrating an outputting operation of photo charges performed in the image sensor with the 2S-4T structure of FIG. 1.
As illustrated in FIG. 1, an image sensor with a 2S-4T structure may include a plurality of unit pixels, each having a photo diode PD (e.g. PD1 PD2, PD3, or PD4) for sensing light to generate a photo charge and four transistors (e.g. a transfer transistor Tx, a reset transistor Rx, an amplification transistor Dx, and a select transistor Sx connected to the photo diodes PD).
The transfer transistors Tx, which are respectively connected to the photo diodes PD1 to PD4, transfer the photo charges generated in the photo diodes PD1 to PD4 to their corresponding floating diffusion regions when they are respectively turned-on. For example, a transfer transistor Tx1 transfers a photo charge generated in a first photo diode PD1 to a floating diffusion region of the first photo diode PD1 when it is turned-on.
The amplification transistors Dx are connected to the transfer transistors Tx (e.g. Tx1 or Tx2) to amplify the photo charges stored in the floating diffusion regions, respectively. The reset transistors Rx are respectively connected to a supply voltage line VDDA or VDDB and the floating diffusion regions. The reset transistors Rx are controlled by a reset signal applied to their gates to reset the respective floating diffusion regions. The select transistors Sx are respectively connected to the amplification transistors Dx and are respectively configured to perform a turn-on operation. When the select transistors Sx are turned-on, the photo charges are outputted through output lines (VoutA or VoutB) extending in a column direction.
FIG. 2 exemplarily illustrates the output of the photo charges performed by the image sensor. The transfer transistors Tx1 of a first photo diode PD1 and a second photo diode PD2 are commonly connected to a first transfer signal line TxA. The transfer transistors Tx2 of a third photo diode PD3 and a fourth photo diode PD4 are commonly connected to a second transfer signal line TxB.
A first select signal may be applied through a first select signal line SxA to the select transistors Sx which is shared by the first photo diode PD1 and the second photo diode PD2. A second select signal may be applied through a second select signal line SxB to the select transistor Sx which is shared by the third photo diode PD3 and the fourth photo diode PD4.
In operation, when the first transfer signal line TxA and the first select signal line SxA are enabled, the photo charge of the first photo diode PD1 is outputted through the first output line VoutA. When the first transfer signal line TxA and the first select signal SxA are enabled, the photo charge of the second photo diode PD2 is outputted through the second output line VoutB. When the second transfer signal line TxB and the first select signal line SxA are enabled, the photo charge of the third photo diode PD3 is outputted through the first output line VoutA. When the second transfer signal line TxB and the first select signal line SxA are enabled, the photo charge of the fourth photo diode PD4 is outputted through the second output line VoutB.
FIG. 2 shows an image sensor in which four unit pixels have their respective photo diodes PD1 to PD4. It may be necessary to provide two output lines VoutA and VoutB and two supply voltage lines VDDA and VDDB. Likewise, if the image sensor includes eight photo diodes, then four output lines may be required.
However, these metal lines including the output lines, the supply voltage lines and other metal lines may affect the photo charge outputs from the photo diodes (e.g. sensitivity of the pixels). For example, the metal lines may shield edges of the photo diodes, which may result in a reduction of the aperture ratio of the photo diodes. As another example, not only the metal lines in columns (e.g. output lines VoutA and VoutB and supply voltage lines VDDA and VDDB) may partially shield the surface of the photo diodes, but the metal lines in rows (e.g. select signal lines SxA and SxB and transfer signal lines TxA and TxB) may also partially shield the surface of the photo diodes. Due to these aspects and others, the sensitivity of all pixels may deteriorate.